**Core Concept**
Uterine atony is the most common cause of postpartum hemorrhage (PPH), accounting for approximately 80% of cases. It occurs when the uterus fails to contract down effectively after delivery, leading to excessive bleeding.

**Why the Correct Answer is Right**
The uterus relies on a complex interplay of hormonal and neural mechanisms to contract down effectively after delivery. The hormone oxytocin plays a crucial role in this process, stimulating uterine contractions through the oxytocin receptor. In the presence of uterine atony, the uterus is unable to contract, and the bleeding continues. Other causes of PPH, such as lacerations or retained placental tissue, can also contribute to bleeding, but uterine atony is the most common underlying cause.

**Why Each Wrong Option is Incorrect**
**Option A:** Retained placental tissue is a possible cause of PPH, but it is not the most common cause. Retained tissue can lead to bleeding due to the presence of the placenta, which can continue to bleed and cause further blood loss.
**Option B:** Uterine rupture is a rare but life-threatening cause of PPH, often associated with uterine surgery or previous uterine trauma. It is not the most common cause of PPH.
**Option C:** Coagulopathy is a rare cause of PPH, often associated with pre-existing bleeding disorders or liver disease. It is not the most common cause of PPH.

**Clinical Pearl / High-Yield Fact**
The "4 Ps" of PPH are a useful mnemonic to remember the common causes: Uterine atony (Pain), Trauma (Placenta), Retained tissue (Placenta), and Coagulopathy (P). This mnemonic can help healthcare providers quickly identify and address the underlying cause of PPH.

**Correct Answer:** A. Uterine atony.